Hip biomechanics in patients with low back pain, what do we know? A systematic review

Background Biomechanical alterations in patients with low back pain (LBP), as reduced range of motion or strength, do not appear to be exclusively related to the trunk. Thus, studies have investigated biomechanical changes in the hip, due to the proximity of this joint to the low back region. However, the relationship between hip biomechanical changes in patients with LBP is still controversial and needs to be summarized. Therefore, the aim of this study was to systematically review observational studies that used biomechanical assessments in patients with non-specific LBP. Methods The search for observational studies that evaluated hip biomechanical variables (i.e., range of motion, kinematic, strength, and electromyography) in adults with non-specific acute, subacute, and chronic LBP was performed in the PubMed, Embase, Cinahl and Sportdiscus databases on February 22nd, 2024. Four blocks of descriptors were used: 1) type of study, 2) LBP, 3) hip and 4) biomechanical assessment. Two independent assessors selected eligible studies and extracted the following data: author, year of publication, country, study objective, participant characteristics, outcomes, and results. The methodological quality of the studies was assessed using the Epidemiological Appraisal Instrument and classified as low, moderate, and high. Due to the heterogeneity of the biomechanical assessment and, consequently, of the results among eligible studies, a descriptive analysis was performed. Results The search strategy returned 338 articles of which 54 were included: nine articles evaluating range of motion, 16 evaluating kinematic, four strength, seven electromyography and 18 evaluating more than one outcome. The studies presented moderate and high methodological quality. Patients with LBP, regardless of symptoms, showed a significant reduction in hip range of motion, especially hip internal rotation, reduction in the time to perform functional activities such as sit-to-stance-to-sit, sit-to-stand or walking, greater activation of the hamstrings and gluteus maximus muscles and weakness of the hip abductor and extensor muscles during specific tests and functional activities compared to healthy individuals. Conclusion Patients with LBP present changes in range of motion, task execution, activation, and hip muscle strength when compared to healthy individuals. Therefore, clinicians must pay greater attention to the assessment and management of the hip during the treatment of these patients. Systematic review registration International Prospective Register of Systematic Reviews (PROSPERO) (CRD42020213599). Supplementary Information The online version contains supplementary material available at 10.1186/s12891-024-07463-5.


Background
Low back pain (LBP) is characterized as pain from below the last ribs to the gluteal margin [1][2][3][4].It can be classified according to the duration of symptoms as acute LBP, lasting up to six weeks, subacute, from six to 12 weeks, or chronic, lasting more than 12 weeks [2].More than 85% of cases do not have a specific cause and, therefore, are diagnosed as non-specific LBP of musculoskeletal origin [3].LBP has affected millions of people over the years, both in high-income countries and in middle-and lowincome countries [5,6].Treatment recommendations for LBP vary depending on their classification [1,3,4,7,8].Physical activity, manual therapy, and supervised exercise are recommendations for patients with acute and subacute LBP [1,4,8,9].For the treatment of chronic nonspecific LBP, the guidelines recommend exercise as the first line of treatment, without specifying the best type of exercise [1][2][3][4]9].
Studies have already been conducted to understand the biomechanical behavior and the changes in the body regions involved during episodes of LBP [10][11][12][13].Biomechanical changes in patients with LBP are not exclusively related to the trunk.The proximity of the hip joint to the lumbar region directed biomechanical investigations to the hip [14][15][16][17][18][19], and several studies on the topic were carried out.These studies investigated the association between hip range of motion and non-specific LBP [19], compared the lower limb muscle strength of patients with LBP to the strength of healthy individuals [15], performed kinematic analysis of the hip during sitting and lifting movements in patients with LBP [14], and investigated the activation of hip muscles in patients with LBP during the standing position [16], movements in the sagittal plane [18]and in the prone hip test [17].These biomechanical studies [14][15][16][17][18][19] showed different results, therefore a summary of their findings would facilitate the understanding of the role of the hip joint in non-specific LBP.Thus, the present study aimed to systematically review observational studies that used biomechanical hip assessment in patients with non-specific LBP.

Study design
Systematic review written according to the guidelines of the Preferred Report Items for Systematic Review and Meta-Analyses (PRISMA) and prospectively registered in the International Prospective Register of Systematic Reviews (CRD42020213599).

Inclusion criteria
We included observational studies, i.e., cross-sectional, cohort, and case-control studies, that performed biomechanical hip assessment in patients with acute, subacute, and chronic non-specific LBP, by measuring muscle strength, range of motion, kinematics, muscle activation, balance, or posture.Patients of both sexes should be over 18 years of age.Studies with pregnant women and patients diagnosed with LBP due to nerve root compromise and severe causes, such as neoplasms, inflammatory diseases, infections, and traumas [3] were not included.In studies that presented data from patients with nonspecific LBP and another diagnosis, only data from patients with non-specific LBP were extracted.Eligible studies had to be published in full in peer-reviewed scientific journals.

Search strategy
The search was performed in the following databases: PubMed, Embase, Cinahl, and Sportdiscus on February 22nd, 2024.The descriptors used were extracted from the Medical Subject Heading (MeSH) and divided into four blocks: 1) type of study, 2) LBP, 3) hip, and 4) biomechanical assessment (Additional file 1).The descriptors were combined to perform the searches with OR between the terms of each block and AND between the blocks.There was no restriction on language and date of publication.

Study selection
Two independent reviewers (GZP and CMNC) conducted the selection process of the studies, first considering the title and abstract, and then the full reading of the study.Disagreements between reviewers were resolved initially by discussion and, in case of persistence, a third reviewer (GCM) reached a consensus.

Data extraction
Data were extracted by two independent reviewers (GZP and GCM) using a customized spreadsheet.The spreadsheet contained bibliometric data such as date of publication of the study, country, language, and authors; objectives of the study; personal and clinical characteristics of the patients such as age, sex, and duration of pain; sample size; description of the type of study; biomechanical variables and description of the assessment; as well as results.Disagreements between reviewers were resolved initially by discussion and, in case of persistence, a third reviewer (CMNC) reached a consensus.

Methodological quality assessment
The methodological quality of the included studies was assessed using the Epidemiological Appraisal Instrument [20].This instrument consists of 43 questions divided into five scales: 1) description, with 17 questions; 2) subject selection, with seven questions; 3) measurement quality, with 10 questions; 4) data analysis, with seven questions; and 5) generalization of results, with two questions.Each question was scored on a scale of 0 to 2, where 0 is "no" or "not informed", 1 is "maybe" or "partial", and 2 is "yes".Questions not applicable to the type of study were disregarded.The scale score was calculated by adding the scores of each question and dividing the total by the number of questions applicable to the type of study used in the assessment.Case-control studies were evaluated with 38 questions, cohort studies with 39 questions, and cross-sectional studies with 34 questions.The methodological quality of the studies was classified as low when the studies had scores between 0 and 0.65, moderate when the studies had scores between 0.7 and 1.35, and high when the studies had scores between 1.4 and 2 [21].The scale score and the methodological quality classification were performed by two independent reviewers (GZP and GCM).In case of disagreement, a third reviewer (CMNC) reached a consensus.

Data analysis
The results extracted from the studies were presented descriptively: mean and standard deviation per group for the case-control and cross-sectional studies and by effect size and confidence interval for the cohort studies.When these data were not presented in the study, two emails with the request were sent to the authors within seven days.If there was no response, the data were presented as provided in the study.The results were grouped by type of biomechanical assessment (range of motion, strength, kinematics, and electromyography) and, later, by type of observational study (cross-sectional, casecontrol, and cohort) and classification of LBP (acute, subacute, and chronic).In studies with more than one type of biomechanical assessment, the outcomes were presented separately in the text considering the biomechanical assessment (some studies were cited more than once throughout the text).Due to the heterogeneity found in the biomechanical hip assessments in patients with LBP, it was not possible to group the results into meta-analyses.

Study selection
The search strategy returned 338 studies: 116 duplicates were excluded and 123 were excluded after reading the titles and abstracts.Of the 99 studies for full-text reading, 47 were excluded: 11 because the participants did not present LBP, nine because they were not observational studies, 12 because they did not perform biomechanical hip assessment, two because they included pregnant women, eight because they included participants under 18 years of age, and five because they were abstracts presented at conferences.The authors of the abstracts were contacted to clarify whether the studies were published in a scientific journal.As a result, one study was added after contact with the authors.Another study was added after manual search.In total, 54 studies were included for data extraction (Fig. 1).

Results of the included studies
Nine inquiries regarding missing descriptive data were sent via email to the authors.Only one study author responded that the requested data were no longer available.Thus, no additional data provided by the authors were included in this systematic review.The objectives of the included studies, characteristics of the sample, outcomes assessed, and the main results are shown in Table 2.
Two studies tested the gluteus medius and maximus muscles during exercise [27,51] and found no differences between patients with chronic LBP and healthy individuals.Two other studies [52,53] tested the hamstring muscles in similar positions (semi-sitting position [53] and sit-to-stand and stand-to-sit [52]) and obtained different results.One study [53] evaluated the semi-sitting position during an isometric contraction and found no difference between patients with chronic LBP and healthy individuals, while other study [52] evaluated the sit-to-stand and stand-to-sit dynamically and observed decreased activation of the time to peak in the hamstring muscles of patients with chronic LBP compared to healthy individuals.

Discussion
The objective of this systematic review was to summarize the results of observational studies that performed biomechanical assessments in patients with non-specific LBP.The 54 studies included in the review used the outcomes range of motion, kinematics, strength, and electromyography for biomechanical assessment.The most common assessments were range of motion and kinematics.Patients with LBP, regardless of the duration of symptoms, showed a significant reduction in hip range of motion, especially total hip rotation [22,25,28,30] and internal hip rotation [19,24,26,30], even with the use of different assessment tools [19,[24][25][26][28][29][30].
Range of motion can be assessed in different ways.The goniometer [26,58,60,65,67,71] and inclinometer [22,24,25,[28][29][30]66] are the most routine, despite having a measurement error between 3 and 5 degrees [73,74].Thus, there may be differences in range of motion, such as between dominant and non-dominant lower limbs [30] and between men and women with subacute and chronic LBP [26], that are not greater than the measurement error in studies assessing this outcome.However, the studies included in this systematic review found a significant reduction in total hip rotation in a variety of patients with LBP from participants of various sports [28,30] to helicopter pilots [25] compared to healthy individuals.Patients with subacute and chronic LBP showed a significant reduction in internal hip rotation compared to healthy individuals [24,26,30], and one study showed an association between LBP and reduced internal hip rotation [19].Kinematics has been extensively studied in patients with LBP, especially during functional activities such as sit-to-stand [36,37,49], walking [38,56,65] and lifting [35,40].Although the current kinematic assessment aims to assess common day-to-day movements [75], no studies have been found that performed kinematic assessment during everyday functional activities that generate pain in patients with LBP, such as putting on shoes or pants [76].In general, patients with chronic LBP showed a reduction in the execution time of functional activities and range of motion, which indicates that they use different strategies than healthy individuals to perform the same functional activities [77].The human movement system has the ability to adapt and use new strategies in the short and long term [75].These strategies adopted by patients with LBP may be the result of motor adaptations to avoid painful movements during the execution of tasks [78,79].This was observed in a previous systematic review [80], that showed "moderate" strength of evidence for reduction of gait preference velocity and "high" strength of evidence for decrease in stride distance in patients with LBP compared to healthy individuals.
The gluteus maximus [17,18,27,51,53,54] and hamstring muscles [17,18,52,53] were the most evaluated in studies that used electromyography as a form of            Hamstring flexibility assessed with the knee straight using a bubble goniometer Hip flexion ROM assessed with the knee bent using a bubble goniometer Joint motions assessed using a Motion Monitor, a magnetic based kinematic system The mean straight leg raise in the participants without LBP was significantly greater than the mean straight leg raise in the LBP group (p < 0.05)          assessment.In addition to being the main extensor muscles of the hip [81], they are superficial, which facilitates electromyographic assessment [51].During the electromyographic assessment, standardization is recommended such as using more than one channel per muscle group and normalizing the value obtained by the maximum voluntary contraction [82,83].However, in patients with LBP, the recommendation is to perform normalization by submaximal voluntary contraction, reducing the chance of interference from the pain intensity [17,52].
The studies included in this systematic review [27,51] did not report the patients' pain intensity, therefore it is not possible to know if there was interference during normalization.However, the gluteus maximus and hamstring muscles had greater electromyographic activation in patients with chronic LBP compared to healthy individuals during the prone hip extension task [17], which may indicate that patients with chronic LBP have difficulty maintaining a stable pelvic lumbar region [17,84].In this systematic review, 9 studies [16, 31-34, 64, 67, 70, 71] evaluated the strength of hip muscles in patients with LBP.Despite the good reliability between the handheld dynamometer and the isokinetic dynamometer [85] and the difference in cost between the devices, the handheld dynamometer is still inaccessible to many health professionals [86][87][88].The cost of the manual dynamometer may be one of the reasons why the included studies assessed strength using a manual muscle test [32] and pressure meter [31,33].The results of this assessment corroborate a recent systematic review [15], in which patients with acute, subacute, and chronic LBP presented weakness of the abductor [32][33][34]71] and extensor muscles [31,34,67,71] of the hip compared to healthy individuals.
Although LBP affects millions of people worldwide [5,6] and some studies seek to understand how the hip behaves in the presence of LBP [14][15][16][17][18][19], this is the first systematic review that summarizes the main findings of biomechanical hip assessments in patients with LBP, considering the type of assessment, the objective of each study, and its result.The results of this systematic review allow an overview of what is expected in the hip assessment of patients with LBP, directing clinicians to more accurate assessments and researchers to new studies that investigate the causes of LBP in a specific population or risk factors in an asymptomatic population.Future research may determine how much the biomechanical outcomes of the hip can be modified during the treatment of patients with LBP, as this question remains unanswered [89].On the other hand, the heterogeneity of the biomechanical assessments and styles of reporting presented a challenge in this systematic review.Although the methodological quality of the included studies was moderate or high, the results were not always presented clearly.

Conclusion
The studies that evaluated the hip biomechanics of patients with LBP are of moderate and high methodological quality.Range of motion is lower in the total, internal, and external hip rotation movements of patients with LBP compared to healthy individuals.The strength of the hip abductor and extensor muscles is lower in patients with LBP compared to asymptomatic individuals.In the kinematic assessment, patients with LBP adopt strategies to reduce speed and change hip flexion movements compared to asymptomatic individuals during functional activities.Patients with LBP submitted to electromyographic assessment presented shorter activation time of the hip muscles and greater amplitude of electromyographic activity compared to healthy individuals.Therefore, greater attention should be given to hip assessment and management during the treatment of these patients.

Fig. 1
Fig. 1 Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines (PRISMA) flow diagram summarizing study selection processes

Table 1
Methodological quality of the included studies

Table 2
Results of the included studies

Range of motion Study Aim Participants' characteristics a Outcomes Result a Nagai et al., 2015 [25] United States
[29]h et al., 2015[29]United States To compare passive ROM of hip extension, hip internal rotation and external rotation and total hip rotation in healthy subjects with active subjects with chronic LBP LBP n = 30, without LBP n = 30 Age LBP: 45.0 ± 12.0 years, without LBP: 34.0 ± 13.1 years Sex: LBP 16 female, 14 male, without LBP 17 female, 13 male

Table 2
(continued) a Van Dillen et al., 2008 [28] United States To examine whether passive hip rotation ROM was different between people with and without LBP who regularly participated in a rotation-related recreational sport LBP n = 24, without LBP n = 24 Age LBP: 26.2 ± 7.3 years, without LBP: 27.0 ± 7.7 years Sex: LBP 7 female, 17 male, without LBP 6 female, 18 male Pain: 2.8 ± 1.8, measured by the Verbal numerical pain intensity rating (0-10) Disability: 15.9 ± 8.3, measured by the Oswestry disability index (0-100%) Cejudo et al., 2020 [22] Spain To analyze the association between hip ROM and LBP and to establish a diagnostic cutoff for ROM of high risk of LBP LBP n = 14, without LBP n = 6 Age: female participants 22.7 ± 3.3 years, male participants 22.3 ± 2.5 years Sex: 10 female, 10 male Passive hip extension, hip adduction, hip flexion with knee flexed and extended, hip abduction, hip internal rotation, hip exter-

Table 2 (continued) Range of motion Study Aim Participants' characteristics a Outcomes Result
[30]k et al., 2020[30]Netherlands To examine whether male judokas with LBP have lower flexibility of their lumbar spinehip complex than judokas without LBP LBP n = 29, without LBP n = 33 Age LBP: 25.1 ± 5.4 years, without LBP: 24.8 ± 5.2 years Sex: 100% male Pain: 31.4 ± 19.1, measured by the Visual analog scale (0-100) Strength Arab et al., 2010 [33] Iran To evaluate the relationship between tightness of iliotibial band and hip abductor muscle strength in subjects with LBP Total n = 300 Age LBP with shortened iliotibial band: 44.2 ± 13.0 years, LBP without shortened iliotibial band: 42.6 ± 14.0 years, without LBP: 43.4 ± 4.4 years Arab et al., 2019 [31] Iran To evaluate whether there is any significant difference in hip extensor strength between participants with and without LBP, and whether these differences could increase the risk of sustaining LBP LBP n = 266, without LBP n = 215 Age LBP: female subjects 44.4 ± 12.4 years, male subjects 42.2 ± 14.4 years, without LBP: female subjects 45.4 ± 13.3 years, male subjects 42.2 ± 15.0 years Sex: LBP 127 female, 139 male, without LBP 82 female, 133 male Cooper et al., 2015 [32] United States To describe the hip abductor weakness in a clinical population with LBP and a matched sample without LBP LBP n = 150, without LBP n = 75 Age LBP: 41.4 ± 13.0 years, without LBP: 40.7 ± 13.9 years Sex: LBP 64.7% female, without LBP 63.5% female

Table 2
(continued) Stability of the upright posture and changes of the gluteus maximus and rectus femoris muscular activity in the lower extremities obtained from a surface EMG and a balance platform Patients with LBP showed differences in the number of fluctuations for the gluteus maximus muscle in the position of eyes opened and eyes closed (number of fluctuations, gluteus maximus without LBP eyes opened 11.9 ± 6.3, LBP group affected limb 6.2 ± 6.9, unaffected limb 7.4 ± 8.2, without LBP eyes closed 10.1 ± 6, LBP group affected limb 5.2 ± 6.8, unaffected limb 7.3 ± 7.2) In the eyes opened and eyes closed positions, a significantly higher average amplitude was recorded from the activity of the rectus femoris muscle on the unaffected side (number of fluctuations, rectus femoris without LBP eyes opened 11.6 ± 7.4, LBP group unaffected limb 5.2 ± 7.1, number of fluctuations, rectus femoris without LBP eyes closed 9.5 ± 6.4, LBP group unaffected limb 4.8 ± 5.6, p < 0.05) ROM Range of motion, LBP low back pain, EMG electromyography, N newtons, CI confidence interval, Kpa kilopascal, s seconds, Nm/kg newton meter per kilogram, ºS −1 degree per second, cm centimeter, ms millisecond, mm/s millimeters per second, m/s meters per second, 3D three dimensions, s seconds, º degree, Hz hertz, kgf/kg kilograms force per kilogram mass, MVIC maximum voluntary isometric contraction, Kg/BMI kilogram normalized to body mass index, μV microvolt a All values are presented as mean ± standard deviation, unless stated